The invention relates to a display device comprising an electro-optical display medium between two supporting plates, a system of pixels arranged in rows and columns, with each pixel being formed by picture electrodes arranged on the facing surfaces of the supporting plates, and a system of row and column electrodes for presenting selection and data signals by means of which a range of voltages dependent on the electro-optical display medium can be presented across the pixels for the purpose of picture display.
The invention also relates to a method of driving such a display device.
A display device of this type is suitable for displaying alphanumerical information and video information by means of passive electro-optical display media such as liquid crystals, electrophoretic suspension and electrochromic materials.
A display device of the type described in the opening paragraph is known from U.S. Pat. No. 4,811,006, in the name of the Applicant. In the device shown in this Application diodes are used as non-linear switching elements in an active matrix, namely two diodes per pixel. Two successive rows of pixels each time have one row electrode in common. The drive mode is such that in television applications (for example with a drive mode in accordance with the PAL or NTSC system) the information of two successive even and odd lines is presented across each pixel at an alternating polarity and at the field frequency. The information of a pixel row is therefore determined by the average signal of two successive even and odd lines. Since each time two rows of picture electrodes are simultaneously written because two successive rows each time have one row electrode in common, such a device provides little flexibility as regards the choice of colour filters to be used. In fact, this choice is limited to strip-shaped colour filters.
It is an object of the invention to provide a picture display device of the type described in the opening paragraph in which the rows of pixels are separately driven without an increase of the number of connections.
It is also an object of the invention to provide such a display device ensuring a considerable freedom in the choice of the colour filters to be used.
The invention is based on the recognition that it is possible to give the pixels a given adjustment per row by charging or discharging the capacitances associated with these pixels after first having discharged or charged them too far (whether or not accurately).
To this end a picture display device according to the invention is characterized in that the device comprises means to apply, prior to selection, an auxiliary voltage across the pixels beyond or on the limit of the voltage range to be used for picture display.
The auxiliary voltage is preferably beyond or on the limit of the range of transition in the transmission/voltage characteristic of the electro-optical medium.
A first preferred embodiment of a display device according to the invention is characterized in that the picture electrode on one of the supporting plates is connected in an electrically conducting manner to the common point of two non-linear switching elements which are arranged in series between a column electrode for data signals and an electrode for the auxiliary voltage.
The auxiliary voltage may then be a fixed reference voltage so that all pixels in a row are first charged negatively or positively to a fixed value and are subsequently charged or discharged to the correct signal value, dependent on the data signals presented.
Since this is effected for each individual row without a subsequent row or previous row being influenced, the picture information can be adapted to a colour filter to be used, which colour filter may be composed of, for example, triplets as described, for example, U.S. Pat. No. 4,908,609 in the name of the Applicant, or it may have, for example a diagonal structure.
Discharging and charging prior to the actual driving operation with the picture information can be effected during the same line period in which the picture information is presented, but also during the preceding line period.
Since each row of pixels is now separately written, the voltage across these pixels can also be inverted per row, which leads to a higher face flicker frequency and hence to a steadier picture.
A second preferred embodiment of a device according to the invention is characterized in that a Zener diode is arranged between a pixel and a row or column electrode. In this case the electrode for the reference voltage may be entirely dispensed with, which leads to a relatively larger portion of the surface of the pixel to be used for the actual display. In addition the number of components in such a device is smaller, which increases the yield manufacturing. Finally the parasitic capacitance is smaller, so that a wider choice of electro-optical materials and the Zener diode to be used is possible.
A method according to the invention is characterized in that prior to presenting a data signal, the pixels are charged positively or negatively to an auxiliary voltage beyond or on the limit of the range to be used for picture display.